1. Field of the Invention
The present invention relates to a high density flexible wiring connection device using a high density flexible flat cable, such as flexible flat cable (hereafter referred to as FFC), including a flexible printed circuit board (hereafter referred to as FPC), which is capable of high-density wiring, and can be attached to and detachable from a socket-type flexible cable connector.
2. Description of the Related Art
There have been advances in size and weight reduction in electronic equipment such cellular phones, personal computers and thin displays. Many electronic components including wiring boards or connection devices used in such electronic equipment use copper or copper alloys as wiring materials. For electrically connecting the electronic components e.g. to other wiring boards or connection devices, connectors such as FPC connectors are often advantageously used because they allow easy size reduction and circuit connection. When such connectors are used for connection, copper wiring terminals are normally subjected to surface treatment by electrolytic plating of e.g. gold or a tin-lead alloy. However, they have problems in that the cost of the gold plating is high, while the plating treatment of a lead-containing alloy may cause lead to dissolve out to contaminate the environment so that lead-free alloys are desired. Thus, studies have been made on pure tin plating and on plating of tin alloys containing no lead.
However, when the pure tin plating or the plating of a tin alloy containing no lead is applied on a copper wiring terminal e.g. of a flexible board to be press-fit into an FPC connector so as to form a plated film on the copper wiring terminal, needle-like crystals called “whiskers” are rapidly formed on the plated film around portions thereof which are pressed by pins of the FPC connector. The whiskers are also formed on various surfaces such as a plated surface of a flexible board, a plated surface of the FPC connector, and a surface of a burr produced by inserting the flexible board into the FPC connector. All such whiskers are considered to be caused by the press-fit force which is generated between the flexible board and the FPC connector, pressing the flexible board from both sides, and which acts as external stresses to each other, i.e. to the flexible board and the FPC connector including the produced burr. The thus formed whiskers may cause short-circuits e.g. between copper wirings, causing failures e.g. in electronic equipment.
A conventional flexible wiring board or connection device uses a resin film to suppress the formation of whiskers to be caused by the press-fit with an FPC connector having pins. For example, assume that a copper wiring terminal of an FPC or an FFC electrolytically plated with pure tin or tin alloy is to be press-fit with a ZIF (Zero Insertion Force) type connector which requires only a small force. For this purpose, it is known to form a thin resin film layer in advance on a portion of the copper wiring terminal which portion is to be electrically connected to a pin of the ZIF type connector so as to suppress the formation of whiskers (refer to e.g. Japanese Laid-open Patent Publication 2005-302575). However, this technology requires that the thin resin film layer be formed with high accuracy on the FPC or FFC copper wiring terminal. Furthermore, there is a risk that the contact resistance of the formed resin film layer to the pin of the ZIF type connector may change and increase due to variations in thickness of the resin film layer. In addition, it is difficult to suppress whiskers formed by repetitive use.
An example of a conventional flexible connector has an insulating sheet with pin contact arrays formed of flexible conductive plates bonded on both sides of the insulating sheet so as to form a dense three-layer structure (refer to e.g. Japanese Laid-open Utility Model Publication Hei 7-11783). Another example of a conventional flexible connector uses an FPC with a base film having a portion which corresponds to the connector, and on the upper and lower surfaces of which connector wirings are formed to be close and parallel to each other so as to form dense electrode arrays (refer to e.g. Japanese Laid-open Utility Model Publication Sho 64-16083).
Yet another example of a conventional flexible connector uses an FPC with a film-based contact portion having two sets of parallel land portions formed thereon as well as lead patters provided on the upper and lower surfaces thereof and connected to the land portions in order to form a flexible board with dense electrode arrays to be used for connector interconnection (refer to e.g. Japanese Laid-open Patent Publication 2001-177206). Also known is a connector to be connected to a flexible wiring board or connection device, in which the connector uses an FPC with an insulating film base having lead patterns printed on upper and lower surfaces thereof to increase the number and density of contacts (refer to e.g. Japanese Laid-open Patent Publication 2003-59560). However, all of these conventional flexible cables or wiring boards or connection devices are aimed at increasing the number and density of contact electrodes, so that they are likely to be affected by whiskers which may cause short-circuits.